Add a //shell:common_lib target. Have mojo_shell_child depend on that.
Add a //shell:common_lib target. Have mojo_shell_child depend on that.

(It turns out we really didn't need much from :lib.)

R=davemoore@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1072683002

jianglu/mojo,chinmaygarde/mojo,afandria/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,chinmaygarde/mojo,collinjackson/mojo,chinmaygarde/mojo,collinjackson/mojo,collinjackson/mojo,jianglu/mojo,chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,collinjackson/mojo,chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,collinjackson/mojo,chinmaygarde/mojo,collinjackson/mojo,collinjackson/mojo,collinjackson/mojo,afandria/mojo,collinjackson/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,afandria/mojo,afandria/mojo